The present invention relates to the production of electrically conductive polypyrrole and is particularly concerned with a process for producing consistent quality oxidant/dopant reagents for converting pyrrole to stable polypyrrole material.
Polypyrrole represents one of the most environmentally stable conducting organic polymers. Its usual preparation has involved electrochemical oxidation of pyrroles in various solvents, with films of the conducting materials depositing on the anode. U.S. Pat. Nos. 4,552,927 to L. F. Warren and 4,582,575 to L. F. Warren, et al, are illustrative of the production of conductive polypyrrole produced in thicker layers and having improved mechanical properties, such as flexibility, as compared to prior art electrochemically produced polypyrrole films.
Chemically prepared polypyrrole has several distinct advantages over electrochemically prepared polypyrrole, especially concerning its potential use commercially. Thus, chemically prepared polypyrrole is much more amenable to scale-up and commercial plant equipment. It also is considerably easier to apply to a substrate or structural material which would then be used in fabricating an end product.
The presently preferred art of preparing conductive polypyrrolye materials chemically, e.g., in the form of powders, films and composites, involves reaction of a pyrrole monomer with salts, e.g., the ferric salts, of the dopant anion to be incorporated in the polymer. However, the dopant anions from suitable commercially available ferric salts, such as the chloride, sulfate, nitrate and perchlorate, often impart inferior physical properties, such as low conductivities and thermal instability to the polypyrrole deposits.
A chemical process for producing conductive polypyrrole having improved conductivity and thermal stability is disclosed in U.S. application Ser. No. 916,917 filed Oct. 9, 1986, of John A. Walker, Edward F. Witucki and Leslie F. Warren, now U.S. Pat. No. 4,697,001, and assigned to the same assignee as the present application. In this patent, a liquid pyrrole is treated with a solution of a strong oxidant/dopant, e.g., ferric alkyl or aryl sulfonate. The preparation of ferric alkyl and aryl sulfonate salts is presently carried out by the reaction of freshly precipitated ferric hydroxide with the corresponding sulfonic acid in aqueous or non-aqueous media, followed by isolation and purification of the resulting ferric sulfonate salts.
However, isolation and purification of the resulting ferric salts are usually very difficult due to their high solubilities, the presence of impurities, both organic and inorganic, and the varying degree of solvation of the ferric ions. The impure solids isolated by evaporation of the solvents from these reactions are typically used as is for reaction with pyrrole to form polypyrrole, with inconsistent results.
An object of the present invention is the preparation of uniform oxidant/dopant reagents for conductive polymer preparation.
Another object is to provide a simplified procedure for the preparation of oxidant/dopant solutions which can be employed in situ for polymerizing a pyrrole to conductive polypyrrole, without isolation and purification of the oxidant/dopant salt.
Yet another object is the provision of procedure as noted above for production of oxidant sulfonate salts, particularly ferric sulfonates, in the form of reagent solutions, which can be employed directly for reaction with pyrrole to produce stable conductive polypyrrole materials of consistent good quality.